1. Technical Field
The present invention relates to a system and method for measuring permeability and pore volume of core samples and, in particular, to an automated system and method therefore for measuring Klinkenberg permeability and pore volume of very tight oil core samples (i.e., permeability of less than one-hundred microdarcies).
2. Discussion of Prior Art
In FIG. 1, is the conventional schematic diagram of a helium flow system in an automated Klinkenberg permeameter/prorsimeter. The apparatus shown in FIG. 1 includes a helium reservoir 10 containing a small helium tank 20 and a large helium tank 30, a manifold 40, a pressure transducer 50, and a core sample holder 60. Valve 80 connects the small helium tank 20 to the manifold 40 over lines 82 and 84. Valve 90 connects the large helium tank 30 over lines 92 and 94 to the manifold 40. The helium that is delivered through valve 130 flows through lines 132 and 134. The vent valve 100 is in fluid communication over lines 102 and 104 with the manifold 40 and the manifold 40 is further connected with the sample holder 60 over lines 112 and 114 through valve 110. Finally, the pressure transducer 50 is connected to the manifold 40 over line 52. Contained within the sample holder 60 is a core plug 70. A poppet valve 120 releases gas to atmosphere as shown.
In conventional operation, the core plug 70 is mounted into a sample holder 60. The core plug 70 is subjected to atmospheric pressure by opening vent valve 100, valve 110, and poppet valve 120. After a period of time, pressure within the core plug 70 reaches equilibrium with atmospheric pressure at zero psig. After reaching atmospheric pressure equilibrium, valves 100, 110, and 120 are closed. The manifold 40 is then pressurized with helium gas by opening fill valve 130 and the manifold is pressurized with helium to a typical value of 250 psig. The pressure in the manifold 40 is monitored by transducer 50. At the desired pressurization, valve 130 is closed. For a period of time, such as a few seconds, the manifold pressure is allowed to stabilize and the pressure in the transducer is recorded.
The system is now ready for testing. Valve 110 is opened and the pressurized helium in manifold 40 is expanded into the sample holder 60 and into the core plug 70. This is further explained by reference to FIG. 2. In FIG. 2, the initial pressurization of the manifold 40 is termed P.sub.FILL. At time t.sub.1, valve 110 is opened and the pressure decreases, in a nonlinear fashion, to an equilibrium pressure, P.sub.E at time t.sub.2. Curve 200 represents the change in pressure due to the increase in volume by adding the volumes from the sample holder 60, i.e., (a) the volume below valve 110 and above the upper surface 400 of core plug 70 and below the sample 70 and above valve 120 is termed the dead volume and (b) within the core plug 70 is termed the pore volume.
FIG. 2 illustrates the pressure-time behavior for four different cases. Curves 210 and 210a depict the approach to pressure equilibrium, P.sub.Ea, for two core plugs having identical pore volumes, but having differing permeability. The plug corresponding to core 210a has a higher permeability than the 210 plug, hence pressure equilibrium is reached more rapidly in the plug for curve 210a. Curves 200 and 200a correspond to core plugs with identical pore volumes, but having volumes which are higher than those depicted by curves 210 and 210a. The higher pore volume allows more gas expansion, which results in a lower equilibrium pressure, P.sub.E. Again, the permeability of the plug represented by curve 200a is higher than that represented by curve 200. Consequently, pressure equilibrium occurs more quickly for plugs having curve 200a than for plugs having curve 200.
Hence, under conventional approaches, when there is no further change of pressure with respect to time, equilibrium has been reached and the pore volume of the core plug 70 can be calculated based on the ratios of the initial pressure P.sub.FILL the equilibrium pressure P.sub.E, and further based upon the known manifold and dead volumes.
For example, if the manifold volume, V.sub.o, is 20.277 cubic centimeters, the dead volume, V.sub.d, is 4.238 cubic centimeters, P.sub.FILL at time t.sub.1 is 238.12 psig, and P.sub.E at time t2 is 162.56 psig, then, the pore volume, V.sub.p can be approximately calculated as: ##EQU1##
There are several drawbacks to the conventional approach for measuring permeability and pore volumes in low permeability (very tight) core plugs (i.e., having a permeability of several microdarcies). Such core samples take longer times (t.sub.1 and t.sub.2) to reach the equilibrium pressures of atmosphere pressure initially in the core plug and P.sub.E and, therefore, the conventional approach is time consuming. Secondly, for any core plug and especially for very tight core plugs, there is no determination (i.e., no way of monitoring) of whether or not the pressure within the plug has truly reached atmospheric pressure, which may take several minutes to an hour or more to obtain. The present invention, as will be explained in the following, eliminates these two major drawbacks and provides a quicker and more accurate determination of pore volume and a quicker determination of permeability for very tight core plugs.